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Okafor opened the scoring in the fifth minute and added a deflected second before the half-hour mark as Manchester United coughed up possession cheaply while trying to play out from the back, and the sending-off of defender Martinez after a video assistant referee (VAR) review looked to have killed off any chance of a comeback.

However, Brazilian midfielder Casemiro had other ideas and he pulled a goal back with a header in the 69th minute, but Leeds keeper Karl Darlow pulled off a string of fine saves as his side put in an enormous defensive effort to ensure that it emerged with the victory.

"It's a massive result for us. We came here to win, that's how we set up" Calvert-Lewin said after the match.

"We executed the game plan and came away with three points. We're delighted."

The home side was returning to action after a long period of inactivity because of the recent international break, but instead of looking fresh and rested, it looked rusty as Leeds pressed hard, repeatedly winning the ball high up the pitch en route to a well-deserved win.

Leeds striker Calvert-Lewin had an early chance only to see his close-range first-time shot hit the home side’s keeper Senne Lammens, and Manchester United failed to heed that warning as the visitors took the lead shortly afterwards, Okafor side-footing home from a cross.

Okafor added a second in the 29th minute as United spurned a number of chances to clear the ball before the midfielder thumped a shot from just outside the box that deflected off Leny Yoro and into the net.

Only a brilliant last-ditch challenge from Martinez prevented the home side from going into the break three goals down as Leeds midfielder Ao Tanaka snapped up yet another loose pass and bore down on goal, but the Argentinean defender swooped in to prevent the ball from crossing the line.

Martinez was then shown a straight red card early in the second half after he was adjudged to have pulled Calvert-Lewin’s hair in a VAR review.

Despite going down to 10 men, Casemiro threw his side a lifeline by pulling a goal back in the 69th minute as Bruno Fernandes sent a deep cross to the back post, and the Brazilian headed it back across the goal to score.

Casemiro went close again in the 85th minute but this time his effort cleared off the line, and a follow-up from Manuel Ugarte was also blocked as the Leeds rearguard held firm to secure the three points.

Manchester United manager Michael Carrick was angry about a number of refereeing decisions, not least the red card for Martinez.

"Second half, I thought the boys, the way they went about it, stayed positive and fought to get something out of it after another shocking, shocking decision to send him [Martinez] off," he said.

"That was one of the worst [decisions] I've seen.

"This is one game, we'll certainly look at it and look to improve. We've got another big game [against Chelsea on Sunday AEST], an important game next week and a big end to the season."

Manchester United remains third on the table with 55 points, ahead of Aston Villa on goal difference, while 15th-placed Leeds put some more daylight between itself and the teams battling relegation by moving up to 36 points, six ahead of Tottenham Hotspur in 18th.

Daniel Farke urged his Leeds United to stay humble and hungry in its quest for Premier League survival.

"Yes. A win is always good for the mood, for confidence. Also for the table," he said when asked if this win would give his team confidence moving forward.

"The reality is that, performance wise, we should already be with far more than 40 points. This team has performed over the whole season with unbelievable consistency.

"We can be more effective in both boxes. For that reality, we are on 36 points. A few more points are needed.

"For now, three points closer, nothing is achieved yet, we have to stay humble, grounded, hungry, we need to make sure we are also on it in the upcoming games to win as many points as possible."

Calvert-Lewin agreed with his boss's assessment of the situation.

"It's important we don't read too much into the external noise and continue to focus on what's in front of us," the striker said.

"It's easy to come here and make history, but it doesn't matter come the end of the season when the points tallies are in.

"That's all we're focussed on – one game after another, making sure that we stay in the Premier League.

"It's a massive result for us. We came here to win, that's how we set up. We executed the game plan and came away with three points. We're delighted."